This one comes from Andrew Price's Blender Guru website. In case you've been living in some other Universe, you use or at least have heard of Cycles. On of its less good features is the time is takes for some simple renderings.
So, get the information right from its source, but I'll post here the main topics either way:
Switch to GPU rendering
Change Tile Size
The one tip I wasn't totally aware yet is the "Change Tile Size". Never thought it could have so much impact.